MUTUAL DEPENDENCE OF PARTS OP ORGANISM. 319 



developed, their nutrition is kept up by the supply of their 

 respective materials which they derive from the blood. Each 

 tissue draws from the circulating current that which it re- 

 quires ] and it is one of the most wonderful proofs of the skill 

 with which the entire fabric has been planned and constructed, 

 that the composition of the blood should be maintained at a 

 nearly uniform standard, in spite of the continual change which 

 is thus taking place in its actual components. It has been 

 justly remarked, that each part of the body, by taking from 

 the blood the peculiar substances which it needs for its own 

 nutrition, does thereby act as an excretory organ, inasmuch 

 as it removes from the blood that which, if retained in it, 

 would be injurious to the nutrition of the body generally. 



383. Hence it seems that such a mutual dependence must 

 exist among the several parts and organs of the body, as 

 causes the evolution of one to supply the conditions requisite 

 for the production of another ; and this view is borne out by 

 a great number of phenomena of very familiar occurrence, 

 which show that a periodical change in one set of organs 

 governs changes in others which at first sight might seem to 

 have no relation to them. Thus the plumage of Birds, at the 

 commencement of the breeding season, becomes (especially in 

 the male) more highly coloured, besides being augmented by 

 the growth of new feathers ; but when the generative organs 

 pass into their condition of periodical inactivity, the plumage 

 begins at once to assume a paler and more sombre hue, and 

 many of the feathers are usually cast, their nutrition being no 

 longer kept up. So, again, it is no uncommon occurrence 

 among Birds, for the female, after ceasing to lay, to assume 

 the plumage of the male, and even to acquire other character- 

 istic parts, as " spurs " in the fowl tribe. That, in these and 

 similar instances, the development of organs is immediately 

 determined by the presence or absence in the blood of the 

 appropriate pabulum for the parts in question, and that its 

 existence depends upon changes taking place in other parts, 

 has been rendered still more probable by the results of expe- 

 riments, which show that if the ordinary changes in one set 

 of organs be prevented by their removal, those usually taking 

 place in the others do not occur. 



384. Though all the tissues derive the materials of their 

 development from the blood which circulates in the vessels, 



